Mechanism of Tungsten Carbide Particle Burn-Off During Hardfacing

Literature Overview

This paper by Qu Shiyao, Wang Xinhong, Zou Zengda, and Liu Xuemei from Shandong University, published in the Welding Journal (Vol. 22, No. 2, 2001, pp. 85–88), investigates the burn-off mechanism of WC hard alloy particles during the hardfacing process. The authors employed scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron probe microanalysis (EPMA) to characterize the melting and burn-off behavior of WC particles in the hardfacing deposit. This work remains highly relevant for engineers involved in surface engineering of cutting tools, mining equipment, and wear-resistant components where WC-based hardfacing is commonly specified.

Core Findings and Technical Interpretation

The study establishes that WC particle burn-off during hardfacing is an unavoidable phenomenon, and understanding its mechanism is critical for optimizing process parameters and maintaining the functional integrity of the deposited layer. The fundamental characteristics of burn-off include: melting of the binder phase at the particle edges, coarsening of carbide particles, and increased inter-particle spacing between single-crystal WC grains.

Burn-Off Modes

The authors identify three primary burn-off modes:

Burn-Off Mode	Description	Primary Mechanism
Edge melting	Melting of the particle periphery	Local overheating at particle-binder interface
Dissolution burn-off	Dissolution of WC into the molten matrix	Thermodynamic driving force for carbon and tungsten dissolution
Diffusion and reaction burn-off	Elemental diffusion and compound formation	Interdiffusion between WC and binder alloy forming new phases

Mechanism Analysis

The core mechanism proposed is as follows: carbides at the edges of WC particles undergo decomposition during the welding thermal cycle. Elemental diffusion occurs between the decomposed carbide and the binder metal, creating a diffusion layer with a concentration gradient. At the outer boundary of this diffusion layer, where atomic concentrations are highest, microstructural transformations occur, forming new compounds that degrade the hardness and wear resistance of the hardfacing deposit.

The degree of burn-off is influenced by several factors:

Hard alloy type: Different WC-Co, WC-Co-Cr, or WC-Ni systems exhibit varying susceptibilities to thermal degradation.
Particle size: Larger particles are more prone to edge melting and dissolution due to their higher thermal mass and longer diffusion paths.
Welding parameters: Heat input, welding speed, and preheating temperature directly affect the thermal exposure of particles.

Engineering Practice Implications

In practical hardfacing operations, the following countermeasures are recommended based on the insights from this study:

Control heat input: Use lower welding currents and higher travel speeds to minimize the thermal exposure of WC particles. A typical parameter window for WC hardfacing using submerged arc welding might be 250–350 A at 400–600 mm/min, depending on wire diameter.
Select appropriate particle size: Particles in the 75–150 μm range generally offer a better balance between burn-off resistance and mechanical interlocking with the binder phase.
Optimize the binder alloy composition: Adding elements such as Cr, Mo, or W to the binder can improve the thermodynamic stability of the interface and reduce dissolution rates.
Preheat and post-heat treatment: Moderate preheating (150–250 °C) reduces thermal gradients and cracking susceptibility, while controlled cooling minimizes residual stress-induced particle dislodgement.

FMEA Perspective on Burn-Off

Failure Mode	Effect	Severity	Cause	Occurrence	Detection	RPN
Excessive particle dissolution	Reduced hardness	8	High heat input	6	5 (requires microanalysis)	240
Binder phase cracking	Particle dislodgement	7	High cooling rate	7	4 (visible inspection)	196
Carbide coarsening	Loss of fine structure	6	Prolonged thermal exposure	5	3 (requires SEM)	90

Study Insights

This paper provides a fundamental understanding of why WC hardfacing deposits often exhibit lower hardness than expected. In my engineering experience, the correlation between observed hardness loss and the three burn-off modes is well established. The diffusion layer formation mechanism is particularly important because it explains why even moderate thermal exposure can degrade performance without visible surface damage. Engineers should always verify the microstructure of critical hardfacing deposits using SEM and EPMA, rather than relying solely on macroscopic hardness measurements. The work underscores that hardfacing is not merely a deposition process but a complex metallurgical transformation that must be carefully controlled to preserve the reinforcing phase integrity.